Internet Engineering Task Force (IETF) S. Esale
Request for Comments: 8223 R. Torvi
Updates: 7473 Juniper Networks
Category: Standards Track L. Jalil
ISSN: 2070-1721 Verizon
U. Chunduri
Huawei
K. Raza
Cisco Systems, Inc.
August 2017
Application-Aware Targeted LDP
Abstract
Recent Targeted Label Distribution Protocol (tLDP) applications, such
as remote Loop-Free Alternates (LFAs) and BGP auto-discovered
pseudowires, may automatically establish a tLDP session with any
Label Switching Router (LSR) in a network. The initiating LSR has
information about the targeted applications to administratively
control initiation of the session. However, the responding LSR has
no such information to control acceptance of this session. This
document defines a mechanism to advertise and negotiate the Targeted
Application Capability (TAC) during LDP session initialization. As
the responding LSR becomes aware of targeted applications, it may
establish a limited number of tLDP sessions for certain applications.
In addition, each targeted application is mapped to LDP Forwarding
Equivalence Class (FEC) elements to advertise only necessary LDP FEC
label bindings over the session. This document updates RFC 7473 for
enabling advertisement of LDP FEC label bindings over the session.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8223.
Esale, et al. Standards Track [Page 1]

RFC 8223 Application-Aware tLDP August 20171. Introduction
LDP uses the Extended Discovery mechanism to establish the
Targeted LDP (tLDP) adjacency and subsequent session, as described in
[RFC5036]. A Label Switching Router (LSR) initiates Extended
Discovery by sending a tLDP Hello to a specific address. The remote
LSR decides to either accept or ignore the tLDP Hello based on local
configuration only. A tLDP application is an application that uses a
tLDP session to exchange information such as FEC label bindings
("FEC" stands for "Forwarding Equivalence Class") with a peer LSR in
the network. For an application such as FEC 128 pseudowire, the
remote LSR is configured with the source LSR address so that it can
use that information to accept or ignore a given tLDP Hello.
However, applications such as remote Loop-Free Alternates (LFAs) and
BGP auto-discovered pseudowires automatically initiate asymmetric
Extended Discovery to any LSR in a network based on local state only.
With these applications, the remote LSR is not explicitly configured
with the source LSR address. So, the remote LSR either responds to
all tLDP Hellos or ignores them.
In addition, since the session is initiated and established after
adjacency formation, the responding LSR has no information on
targeted applications available from which it can choose a session
with a targeted application that it is configured to support. Also,
the initiating LSR may employ a limit per application on locally
initiated automatic tLDP sessions; however, the responding LSR has
no such information to employ a similar limit on the incoming tLDP
sessions. Further, the responding LSR does not know whether the
source LSR is establishing a tLDP session for configured
applications, automatic applications, or both.
This document proposes and describes a solution to advertise the
Targeted Application Capability (TAC), consisting of a list of
targeted applications, during initialization of a tLDP session. It
also defines a mechanism to enable a new application and disable an
old application after session establishment. This capability
advertisement provides the responding LSR with the necessary
information to control the acceptance of tLDP sessions
per application. For instance, an LSR may accept all BGP
auto-discovered tLDP sessions as described in [RFC6074] but may only
accept a limited number of remote LFA tLDP sessions as described
in [RFC7490].
Also, the tLDP application is mapped to LDP FEC element types to
advertise specific application FECs only, avoiding the advertisement
of other unnecessary FECs over a tLDP session.
Esale, et al. Standards Track [Page 3]

RFC 8223 Application-Aware tLDP August 2017
ignore the duplicate elements. If the receiver LSR receives an
unknown TA-Id in the TAE, it MUST silently ignore such a TAE and
continue processing the rest of the TLV.
If the receiver LSR does not receive the TAC TLV in the
Initialization message or it does not understand the TAC TLV, the TAC
negotiation is considered unsuccessful and the session establishment
proceeds as per [RFC5036]. On receipt of a valid TAC TLV, an LSR
MUST generate its own TAC TLV with TAEs consisting of unique TA-Ids
that it supports over the tLDP session. If there is at least one
common TAE between the TAC TLV it has received and its own, the
session MUST proceed to establishment as per [RFC5036]. If not, an
LSR MUST send a 'Session Rejected/Targeted Application Capability
Mismatch' Notification message to the peer and close the session.
The initiating LSR SHOULD tear down the corresponding tLDP adjacency
after sending or receiving a 'Session Rejected/Targeted Application
Capability Mismatch' Notification message to or from the responding
LSR, respectively.
If both of the peers support the TAC TLV, an LSR decides to establish
or close a tLDP session based on the negotiated list of targeted
applications. For example, an initiating LSR advertises A, B, and C
as TA-Ids, and the responding LSR advertises C, D, and E as TA-Ids.
Then, the negotiated TA-Id as per both LSRs is C. In another
example, an initiating LSR advertises A, B, and C as TA-Ids, and the
responding LSR, which acts as a passive LSR, advertises all of the
applications -- A, B, C, D, and E -- as TA-Ids that it supports over
this session. The negotiated targeted applications as per both LSRs
are then A, B, and C. Finally, if the initiating LSR advertises A,
B, and C as TA-Ids and the responding LSR advertises D and E as
TA-Ids, then the negotiated targeted applications as per both LSRs
are "none". Therefore, if the intersection of the sets of received
and sent TA-Ids is null, then the LSR sends a 'Session
Rejected/Targeted Application Capability Mismatch' Notification
message to the peer LSR and closes the session.
When the responding LSR playing the active role [RFC5036] in LDP
session establishment receives a 'Session Rejected/Targeted
Application Capability Mismatch' Notification message, it MUST set
its session setup retry interval to a maximum value -- that is,
0xFFFF. The session MAY stay in a non-existent state. When it
detects a change in the initiating LSR or local LSR configuration
pertaining to the TAC TLV, it MUST clear the session setup backoff
delay associated with the session to reattempt session establishment.
An LSR detects the configuration change on the other LSR upon receipt
of a tLDP Hello message that has a higher configuration sequence
number than the earlier tLDP Hello message.
Esale, et al. Standards Track [Page 6]

RFC 8223 Application-Aware tLDP August 2017
When the initiating LSR playing the active role in LDP session
establishment receives a 'Session Rejected/Targeted Application
Capability Mismatch' Notification message, it MUST either (1) close
the session and tear down the corresponding tLDP adjacency or (2) set
its session setup retry interval to a maximum value -- that is,
0xFFFF.
If the initiating LSR decides to tear down the associated tLDP
adjacency, the session is closed on the initiating LSR as well as the
responding LSR. It MAY also take appropriate actions. For instance,
if an automatic session intended to support the remote LFA
application is rejected by the responding LSR, the initiating LSR may
inform the IGP to calculate another PQ node [RFC7490] for the route
or set of routes. More specific actions are a local matter and are
outside the scope of this document.
If the initiating LSR sets the session setup retry interval to
maximum, the session MAY stay in a non-existent state. When this LSR
detects a change in the responding LSR configuration or its own
configuration pertaining to the TAC TLV, it MUST clear the session
setup backoff delay associated with the session in order to reattempt
session establishment.
After a tLDP session using the TAC mechanism has been established,
the initiating and responding LSRs MUST distribute FEC label bindings
for the negotiated applications only. For instance, if the tLDP
session is established for a BGP auto-discovered pseudowire, only FEC
129 label bindings MUST be distributed over the session. Similarly,
an LSR operating in downstream on-demand mode MUST request FEC label
bindings for the negotiated applications only.
If the TAC and the Dynamic Capability [RFC5561] are negotiated during
session initialization, the TAC MAY be renegotiated after session
establishment by sending an updated TAC TLV in the LDP Capability
message. The updated TAC TLV carries TA-Ids with an incremental
update only. The updated TLV MUST consist of one or more TAEs with
the E-bit set (1) or off (0), to advertise or withdraw the new
application and the old application, respectively. This may lead to
advertisements or withdrawals of certain types of FEC label bindings
over the session or to teardown of the tLDP adjacency and,
subsequently, the session.
The TAC is advertised on the tLDP session only. If the tLDP session
changes to a link session, an LSR SHOULD withdraw it with the S-bit
set to 0. Similarly, if the link session changes to tLDP, an LSR
SHOULD advertise it via the Capability message. If the capability
negotiation fails, this may lead to destruction of the tLDP session.
Esale, et al. Standards Track [Page 7]

RFC 8223 Application-Aware tLDP August 2017
By default, an LSR SHOULD accept tLDP Hellos in order to then accept
or reject the tLDP session based on the application information.
In addition, an LSR SHOULD allow the configuration of any TA-Id in
order to facilitate the use of private TA-Ids by a network operator.
2.3. LDP Message Procedures2.3.1. Initialization Message
1. The S-bit of the TAC TLV MUST be set to 1 to advertise the TAC and
SHOULD be ignored on receipt, as described in [RFC5561].
2. The E-bit of the TAE MUST be set to 1 to enable the targeted
application and SHOULD be ignored on receipt.
3. An LSR MAY add the State Advertisement Control Capability by
mapping the TAE to the State Advertisement Control (SAC) elements
as defined in Section 4.
2.3.2. Capability Message
After a change to local configuration, the initiating or responding
LSR may renegotiate the TAC via the Capability message.
1. The S-bit of the TAC is set to 1 or 0 to advertise or withdraw it.
2. After the configuration change, if there is no common TAE between
its new TAE list and the peer's TAE list, the LSR MUST send a
'Session Rejected/Targeted Application Capability Mismatch'
Notification message and close the session.
3. If there is a common TAE, an LSR MAY also update the SAC
Capability based on the updated TAC, as described in Section 4,
and send the updated TAC and SAC Capability in a Capability
message to the peer.
4. A receiving LSR processes the Capability message with the TAC TLV.
If the S-bit is set to 0, the TAC is disabled for the session.
5. If the S-bit is set to 1, the LSR processes a list of TAEs from
the TAC's data with the E-bit set to 1 or 0 to update the
peer's TAE.
Esale, et al. Standards Track [Page 8]

RFC 8223 Application-Aware tLDP August 2017
+----------------------+------------------------+------------------+
| | | P2MP |
|mLDP Node Protection | mLDP node protection | MP2MP-up |
| | | MP2MP-down |
| | | HSMP-downstream |
| | | HSMP-upstream |
+----------------------+------------------------+------------------+
| | | |
|IPv4 intra-area FECs* | IPv4 intra-area FECs* | IPv4 prefix |
+----------------------+------------------------+------------------+
| | | |
|IPv6 intra-area FECs* | IPv6 intra-area FECs* | IPv6 prefix |
+----------------------+------------------------+------------------+
* Intra-area FECs: FECs that are on the shortest-path tree and
are not leafs of the shortest-path tree.
4. Interaction of Targeted Application Capabilities and State Advertisement Control Capabilities
As described in this document, the set of TAEs negotiated between two
LDP peers advertising the TAC represents the willingness of both
peers to advertise state information for a set of applications. The
set of applications negotiated by the TAC mechanism is symmetric
between the two LDP peers. In the absence of further mechanisms, two
LDP peers will both advertise state information for the same set of
applications.
As described in [RFC7473], the SAC TLV can be used by an LDP speaker
to communicate its interest or disinterest in receiving state
information from a given peer for a particular application. Two LDP
peers can use the SAC mechanism to create asymmetric advertisements
of state information between the two peers.
The TAC negotiation facilitates the awareness of targeted
applications to both of the peers. It enables them to advertise only
necessary LDP FEC label bindings corresponding to negotiated
applications. With the SAC, the responding LSR is not aware of
targeted applications. Thus, it may be unable to communicate its
interest or disinterest in receiving state information from the peer.
Therefore, when the responding LSR is not aware of targeted
applications such as remote LFAs and BGP auto-discovered pseudowires,
the TAC mechanism should be used, and when the responding LSR is
aware (with appropriate configuration) of targeted applications such
as FEC 128 pseudowire, the SAC mechanism should be used. Also, after
the TAC mechanism makes the responding LSR aware of targeted
applications, the SAC mechanism may be used to communicate its
Esale, et al. Standards Track [Page 10]

RFC 8223 Application-Aware tLDP August 2017
disinterest in receiving state information from the peer for a
particular negotiated application, creating asymmetric
advertisements.
Thus, the TAC mechanism enables two LDP peers to symmetrically
advertise state information for negotiated targeted applications.
Further, the SAC mechanism enables both of them to asymmetrically
disable receipt of state information for some of the already-
negotiated targeted applications. Collectively, the TAC mechanism
and the SAC mechanism can both be used to control the FEC label
bindings that are advertised over the tLDP session. For instance,
suppose that the initiating LSR establishes a tLDP session, using the
TAC mechanism, with the responding LSR for remote LFA and FEC 129 PW
targeted applications. So, each LSR advertises the corresponding FEC
label bindings. Further, suppose that the initiating LSR is not the
PQ node for the responding LSR's remote LFA IGP calculations. In
such a case, the responding LSR may use the SAC mechanism to convey
its disinterest in receiving state information for remote LFA tLDP
applications.
For a given tLDP session, the TAC mechanism can be used without the
SAC mechanism, and the SAC mechanism can be used without the TAC
mechanism. It is useful to discuss the behavior that occurs when the
TAC and SAC mechanisms are used on the same tLDP session. The TAC
mechanism MUST take precedence over the SAC mechanism with respect to
enabling applications for which state information will be advertised.
For a tLDP session using the TAC mechanism, the LDP peers MUST NOT
advertise state information for an application that has not been
negotiated in the most recent TAE list (referred to as a
non-negotiated application). This is true even if one of the peers
announces its interest in receiving state information that
corresponds to the non-negotiated application by sending a SAC TLV.
In other words, when the TAC mechanism is being used, the SAC
mechanism cannot and should not enable state information
advertisements for applications that have not been enabled by the TAC
mechanism.
On the other hand, the SAC mechanism MUST take precedence over the
TAC mechanism with respect to disabling state information
advertisements. If an LDP speaker has announced its disinterest in
receiving state information for a given application to a given peer
using the SAC mechanism, its peer MUST NOT send state information for
that application, even if the two peers have negotiated the
corresponding application via the TAC mechanism.
Esale, et al. Standards Track [Page 11]

RFC 8223 Application-Aware tLDP August 2017
For the purposes of determining the correspondence between targeted
applications defined in this document and application state as
defined in [RFC7473], an LSR MUST use the following mappings:
LDPv4 Tunneling - IPv4 Prefix-LSPs
LDPv6 Tunneling - IPv6 Prefix-LSPs
LDPv4 Remote LFA - IPv4 Prefix-LSPs
LDPv6 Remote LFA - IPv6 Prefix-LSPs
LDP FEC 128 PW - FEC 128 P2P-PW
LDP FEC 129 PW - FEC 129 P2P-PW
An LSR MUST map the targeted application to the LDP capability
as follows:
mLDP Tunneling - P2MP Capability, MP2MP Capability, and HSMP LSP
Capability TLV
mLDP Node Protection - P2MP Capability, MP2MP Capability, and HSMP
LSP Capability TLV
5. Use Cases5.1. Remote LFA Automatic Targeted Session
The LSR determines that it needs to form an automatic tLDP session
with a remote LSR based on IGP calculation as described in [RFC7490]
or some other mechanism outside the scope of this document. The LSR
forms the tLDP adjacency and constructs an Initialization message
with the TAC TLV consisting of the TAE as the remote LFA during
session establishment. The receiver LSR processes the LDP
Initialization message and verifies whether it is configured to
accept a remote LFA tLDP session. If it is, it may further verify
that establishing such a session does not exceed the configured limit
for remote LFA sessions. If all of these conditions are met, the
receiver LSR may respond back with an Initialization message with the
TAC corresponding to the remote LFA, and subsequently the session
may be established.
After the session using the TAC mechanism has been established, the
sender and receiver LSRs distribute IPv4 or IPv6 FEC label bindings
over the session. Further, the receiver LSR may determine that it
does not need these FEC label bindings. So, it may disable the
receipt of these FEC label bindings by mapping the TAE to the State
Advertisement Control Capability as described in Section 4.
Esale, et al. Standards Track [Page 12]

RFC 8223 Application-Aware tLDP August 20175.2. FEC 129 Auto-discovery Targeted Session
BGP auto-discovery may determine whether the LSR needs to initiate an
auto-discovery tLDP session with a border LSR. Multiple LSRs may try
to form an auto-discovered tLDP session with a border LSR. So, a
service provider may want to limit the number of auto-discovered tLDP
sessions that a border LSR can accept. As described in Section 2,
LDP may convey targeted applications with the TAC TLV to a border
LSR. A border LSR may establish or reject the tLDP session based on
local administrative policy. Also, as the receiver LSR becomes aware
of targeted applications, it can also employ an administrative policy
for security. For instance, it can employ a policy to accept all
auto-discovered sessions from a source addresses list.
Moreover, the sender and receiver LSRs must exchange FEC 129 label
bindings only over the tLDP session.
5.3. LDP over RSVP and Remote LFA Targeted Session
An LSR may want to establish a tLDP session with a remote LSR for
LDP-over-RSVP tunneling and remote LFA applications. The sender LSR
may add both of these applications as a unique TAE in the TAC data of
a TAC TLV. The receiver LSR may have reached a configured limit for
accepting remote LFA automatic tLDP sessions, but it may have been
configured to accept LDP-over-RSVP tunneling. In such a case, the
tLDP session is formed for both LDP-over-RSVP tunneling and remote
LFA applications, as both need the same FECs -- IPv4, IPv6, or both.
5.4. mLDP Node Protection Targeted Session
A Merge Point (MPT) LSR may determine that it needs to form an
automatic tLDP session with the upstream point of local repair (PLR)
LSR for MP2P and MP2MP LSP [RFC6388] node protection as described in
[RFC7715]. The MPT LSR may add a new tLDP application -- mLDP
protection -- as a unique TAE in the TAC data of a TAC TLV and send
it in the Initialization message to the PLR. If the PLR is
configured for mLDP node protection and establishing this session
does not exceed the limit of either mLDP node protection sessions or
automatic tLDP sessions, the PLR may decide to accept this session.
Also, the PLR may respond back with the Initialization message with a
TAC TLV that has one of the TAEs as mLDP protection, and the session
proceeds to establishment as per [RFC5036].
Esale, et al. Standards Track [Page 13]

RFC 8223 Application-Aware tLDP August 20176. Security Considerations
The procedures described in this document do not introduce any
changes to LDP security considerations as described in [RFC5036].
As described in [RFC5036], DoS attacks via Extended Hellos, which are
required to establish a tLDP session, can be addressed by filtering
Extended Hellos using access lists that define addresses with which
Extended Discovery is permitted. Further, as described in
Section 5.2 of this document, an LSR can employ a policy to accept
all auto-discovered Extended Hellos from the configured source
addresses list.
Also, for the two LSRs supporting the TAC, the tLDP session is only
established after successful negotiation of the TAC. The initiating
and receiving LSRs MUST only advertise TA-Ids that they support --
in other words, what they are configured for over the tLDP session.
7. IANA Considerations
IANA has assigned the following code point for the new Capability
Parameter TLV defined in this document. The code point has been
assigned from the "TLV Type Name Space" sub-registry of the "Label
Distribution Protocol (LDP) Parameters" registry.
Value Description Reference
------ ------------------------------- ---------
0x050F Targeted Application Capability RFC 8223
IANA has assigned a new status code from the "Status Code Name Space"
sub-registry of the "Label Distribution Protocol (LDP) Parameters"
registry.
Value E Description Reference
---------- --- ----------------------------------- ---------
0x0000004C 1 Session Rejected/Targeted
Application Capability Mismatch RFC 8223Esale, et al. Standards Track [Page 14]